In December, officials from across the world will gather in Paris, France, to try to hammer out a deal to tackle global warming. Here's what we know and don't know about the Earth's changing climate.
What is climate change?
The planet's climate has constantly been changing over geological time. The global average temperature today is about 15C, though geological evidence suggests it has been much higher and lower in the past.
However, the current period of warming is occurring more rapidly than many past events. Scientists are concerned that the natural fluctuation, or variability, is being overtaken by a rapid human-induced warming that has serious implications for the stability of the planet's climate.
What is the "greenhouse effect"?
The greenhouse effect refers to the way the Earth's atmosphere traps some of the energy from the Sun. Solar energy radiating back out to space from the Earth's surface is absorbed by atmospheric greenhouse gases and re-emitted in all directions.
The energy that radiates back down to the planet heats both the lower atmosphere and the surface. Without this effect, the Earth would be about 30C colder, making our planet hostile to life.
Scientists believe we are adding to the natural greenhouse effect with gases released from industry and agriculture (known as emissions), trapping more energy and increasing the temperature. This is commonly referred to as global warming or climate change.
The most important of these greenhouse gases in terms of its contribution to warming is water vapour, but concentrations show little change and it persists in the atmosphere for only a few days.
On the other hand, carbon dioxide (CO2) persists for much longer (it would take hundreds of years for it to return to pre-industrial levels). In addition, there is only so much CO2 that can be soaked up by natural reservoirs such as the oceans.
Most man-made emissions of CO2 are through the burning of fossil fuels, as well as through cutting down carbon-absorbing forests. Other greenhouse gases such as methane and nitrous oxide are also released through human activities, but their overall abundance is small compared with carbon dioxide.
Since the industrial revolution began in 1750, CO2 levels have risen by more than 30% and methane levels have risen more than 140%. The concentration of CO2 in the atmosphere is now higher than at any time in at least 800,000 years.
NASAWhat is the evidence for warming?
Temperature records going back to the late 19th Century show that the average temperature of the Earth's surface has increased by about 0.8C (1.4F) in the last 100 years. About 0.6C (1.0F) of this warming occurred in the last three decades.
Satellite data shows an average increase in global sea levels of some 3mm per year in recent decades. A large proportion of the change in sea level is accounted for by the thermal expansion of seawater. As seawater warms up, the molecules become less densely packed, causing an increase in the volume of the ocean.
But the melting of mountain glaciers and the retreat of polar ice sheets are also important contributors. Most glaciers in temperate regions of the world and along the Antarctic Peninsula are in retreat. Since 1979, satellite records show a dramatic decline in Arctic sea-ice extent, at an annual rate of 4% per decade. In 2012, the ice extent reached a record minimum that was 50% lower than the 1979-2000 average.
The Greenland Ice Sheet has experienced record melting in recent years; if the entire 2.8 million cu km sheet were to melt, it would raise sea levels by 6m.
Satellite data shows the West Antarctic Ice Sheet is also losing mass, and a recent study indicated that East Antarctica, which had displayed no clear warming or cooling trend, may also have started to lose mass in the last few years. But scientists are not expecting dramatic changes. In some places, mass may actually increase as warming temperatures drive the production of more snows.
The effects of a changing climate can also be seen in vegetation and land animals. These include earlier flowering and fruiting times for plants and changes in the territories (or ranges) occupied by animals.
SPLWhat about the pause?
In the last few years, there has been a lot of talk about a pause in global warming. Commentators argued that since 1998, there had been no significant global warming despite ever increasing amounts of carbon dioxide being emitted. Scientists have tried to explain this in a number of ways.
These include:
- variations in the Sun's energy output
- a decline in atmospheric water vapour
- greater storage of heat by the oceans.
But so far, there is no general consensus on the precise mechanism behind the pause.
Sceptics highlight the pause as an example of the fallibility of predictions based on computer climate models. On the other hand, climate scientists point out that the hiatus occurs in just one component of the climate system - the global mean surface temperature - and that other indicators, such as melting ice and changes to plant and animal life, demonstrate that the Earth has continued to warm.
How much will temperatures rise in future?
In its 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC) forecast a global temperature increase of between 1.8C and 4C by 2100.
Even if we cut greenhouse gas emissions dramatically now, scientists say the effects will continue because parts of the climate system, particularly large bodies of water and ice, can take hundreds of years to respond to changes in temperature. It also takes greenhouse gases decades to be removed from the atmosphere.
How will climate change affect us?
The scale of potential impacts is uncertain. The changes could drive freshwater shortages, bring sweeping changes in food production conditions, and increase the number of deaths from floods, storms, heat waves and droughts. This is because climate change is expected to increase the frequency of extreme weather events - though linking any single event to global warming is complicated.
Scientists forecast more rainfall overall, but say the risk of drought in inland areas during hot summers will increase. More flooding is expected from storms and rising sea levels. There are, however, likely to be very strong regional variations in these patterns.
Poorer countries, which are least equipped to deal with rapid change, could suffer the most.
Plant and animal extinctions are predicted as habitats change faster than species can adapt, and the World Health Organization (WHO) has warned that the health of millions could be threatened by increases in malaria, water-borne disease and malnutrition.
As an increased amount of CO2 is released into the atmosphere, there is increased uptake of CO2 by the oceans, and this leads to them becoming more acidic. This ongoing process of acidification could pose major problems for the world's coral reefs, as the changes in chemistry prevent corals from forming a calcified skeleton, which is essential for their survival.
Computer models are used to study the dynamics of the Earth's climate and make projections about future temperature change. But these climate models differ on "climate sensitivity" - the amount of warming or cooling that occurs as a particular factor, such as CO2. goes up or down.
Models also differ in the way that they express "climate feedbacks".
Global warming will cause some changes that look likely to create further heating, such as the release of large quantities of the greenhouse gas methane as permafrost (permanently frozen soil found mainly in the Arctic) melts. This is known as a positive climate feedback.
But negative feedbacks exist that could offset warming. Various "reservoirs" on Earth absorb CO2 as part of the carbon cycle - the process through which carbon is exchanged between, for example, the oceans and the land.
The question is: how will these balance out?
Font: http://www.bbc.com/news/science-environment-24021772
A brief history of climate change
1712 - British
ironmonger Thomas Newcomen invents the first widely used steam engine, paving
the way for the Industrial Revolution and industrial scale use of coal.
I The Newcomen Engine
foreshadowed industrial scale use of coal
1800 - World
population reaches one billion.
1824 - French
physicist Joseph Fourier describes the Earth's natural "greenhouse
effect". He writes: "The temperature [of the Earth] can be augmented
by the interposition of the atmosphere, because heat in the state of light
finds less resistance in penetrating the air, than in re-passing into the air
when converted into non-luminous heat."
1861 - Irish
physicist John Tyndall shows that water vapour and certain other gases create
the greenhouse effect. "This aqueous vapour is a blanket more necessary to
the vegetable life of England than clothing is to man," he concludes. More
than a century later, he is honoured by having a prominent UK climate research
organisation - the Tyndall Centre - named after him.
1886 - Karl
Benz unveils the Motorwagen, often regarded as the first true automobile.
1896 - Swedish
chemist Svante Arrhenius concludes that industrial-age coal burning will
enhance the natural greenhouse effect. He suggests this might be beneficial for
future generations. His conclusions on the likely size of the "man-made
greenhouse" are in the same ballpark - a few degrees Celsius for a
doubling of CO2 - as modern-day climate models.
ISvante Arrhenius unlocked the man-made greenhouse a century ago
1900 - Another Swede, Knut Angstrom, discovers that even at
the tiny concentrations found in the atmosphere, CO2 strongly absorbs parts of
the infrared spectrum. Although he does not realize the significance, Angstrom
has shown that a trace gas can produce greenhouse warming.
1927 - Carbon
emissions from fossil fuel burning and industry reach one billion tonne per
year.
1930 - Human
population reaches two billion.
1938 - Using
records from 147 weather stations around the world, British engineer Guy Callendar
shows that temperatures had risen over the previous century. He also shows that
CO2 concentrations had increased over the same period, and suggests this caused
the warming. The "Callendar effect" is widely dismissed by
meteorologists.
1955 - Using a new generation of equipment including early
computers, US researcher Gilbert Plass analyses in detail the infrared
absorption of various gases. He concludes that doubling CO2 concentrations
would increase temperatures by 3-4C.
1957 - US
oceanographer Roger Revelle and chemist Hans Suess show that seawater will not
absorb all the additional CO2 entering the atmosphere, as many had assumed.
Revelle writes: "Human beings are now carrying out a large scale
geophysical experiment..."
1958 - Using
equipment he had developed himself, Charles David (Dave) Keeling begins
systematic measurements of atmospheric CO2 at Mauna Loa in Hawaii and in
Antarctica. Within four years, the project - which continues today - provides
the first unequivocal proof that CO2 concentrations are rising.
Change in future is likely to be more fundamental and
more widespread than anything we have known hither to Margaret Thatcher, Former
UK Prime Minister
1960 - Human
population reaches three billion.
1965 - A US President's Advisory Committee panel warns that
the greenhouse effect is a matter of "real concern".
1972 - First UN
environment conference, in Stockholm. Climate change hardly registers on the
agenda, which centers on issues such as chemical pollution, atomic bomb testing
and whaling. The United Nations Environment Programme (Unep) is formed as a
result.
1975 - Human
population reaches four billion.
1975 - US
scientist Wallace Broecker puts the term "global warming" into the
public domain in the title of a scientific paper.
1987 - Human
population reaches five billion
1987 - Montreal
Protocol agreed, restricting chemicals that damage the ozone layer. Although
not established with climate change in mind, it has had a greater impact on
greenhouse gas emissions than the Kyoto Protocol.
1988 -
Intergovernmental Panel on Climate Change (IPCC) formed to collate and assess
evidence on climate change.
1989 - UK Prime
Minister Margaret Thatcher - possessor of a chemistry degree - warns in a
speech to the UN that "We are seeing a vast increase in the amount of
carbon dioxide reaching the atmosphere... The result is that change in future
is likely to be more fundamental and more widespread than anything we have
known hitherto." She calls for a global treaty on climate change.
1989 - Carbon
emissions from fossil fuel burning and industry reach six billion tonnes per
year.
The CO2 concentration, as measured at Mauna Loa, has risen steadily
1990 - IPCC
produces First Assessment Report. It concludes that temperatures have risen by
0.3-0.6C over the last century, that humanity's emissions are adding to the
atmosphere's natural complement of greenhouse gases, and that the addition
would be expected to result in warming.
1992 - At the
Earth Summit in Rio de Janeiro, governments agree the United Framework
Convention on Climate Change. Its key objective is "stabilization of
greenhouse gas concentrations in the atmosphere at a level that would prevent
dangerous anthropogenic interference with the climate system". Developed countries
agree to return their emissions to 1990 levels.
1995 - IPCC
Second Assessment Report concludes that the balance of evidence suggests
"a discernible human influence" on the Earth's climate. This has been
called the first definitive statement that humans are responsible for climate
change.
1997 - Kyoto
Protocol agreed. Developed nations pledge to reduce emissions by an average of
5% by the period 2008-12, with wide variations on targets for individual
countries. US Senate immediately declares it will not ratify the treaty.
1998 - Strong
El Nino conditions combine with global warming to produce the warmest year on
record. The average global temperature reached 0.52C above the mean for the
period 1961-90 (a commonly used baseline).
1998 -
Publication of the controversial "hockey stick" graph indicating that
modern-day temperature rise in the northern hemisphere is unusual compared with
the last 1,000 years. The work would later be the subject of two enquiries
instigated by the US Congress.
Rajendra Pachauri's IPCC netted
the Nobel Peace Prize in 2007
1999 - Human
population reaches six billion.
2001 -
President George W Bush removes the US from the Kyoto process.
2001 - IPCC
Third Assessment Report finds "new and stronger evidence" that
humanity's emissions of greenhouse gases are the main cause of the warming seen
in the second half of the 20th Century.
2005 - The
Kyoto Protocol becomes international law for those countries still inside it.
2005 - UK Prime
Minister Tony Blair selects climate change as a priority for his terms as chair
of the G8 and president of the EU.
2006 - The
Stern Review concludes that climate change could damage global GDP by up to 20%
if left unchecked - but curbing it would cost about 1% of global GDP.
2006 - Carbon
emissions from fossil fuel burning and industry reach eight billion tonnes per
year.
2007 - The
IPCC's Fourth Assessment Report concludes it is more than 90% likely that
humanity's emissions of greenhouse gases are responsible for modern-day climate
change.
2007 - The IPCC
and former US vice-president Al Gore receive the Nobel Peace Prize "for
their efforts to build up and disseminate greater knowledge about man-made
climate change, and to lay the foundations for the measures that are needed to
counteract such change".
2007 - At UN
negotiations in Bali, governments agree the two-year "Bali roadmap"
aimed at hammering out a new global treaty by the end of 2009. 2008 -
Half a century after beginning observations at Mauna Loa, the Keeling project
shows that CO2 concentrations have risen from 315 parts per million (ppm) in
1958 to 380ppm in 2008.
2008 - Two
months before taking office, incoming US president Barack Obama pledges to
"engage vigorously" with the rest of the world on climate change.
2009 - China
overtakes the US as the world's biggest greenhouse gas emitter - although the
US remains well ahead on a per-capita basis.
2009 - Computer
hackers download a huge tranche of emails from a server at the University of
East Anglia's Climatic Research Unit and release some on the internet, leading
to the "ClimateGate" affair.
2009 - 192
governments convene for the UN climate summit in Copenhagen with expectations
of a new global agreement high; but they leave only with a controversial
political declaration, the Copenhagen Accord.
2010 -
Developed countries begin contributing to a $30bn, three-year deal on
"Fast Start Finance" to help them "green" their economies
and adapt to climate impacts.
2010 - A series
of reviews into "ClimateGate" and the IPCC ask for more openness, but
clear scientists of malpractice.
2010 - The UN
summit in Mexico does not collapse, as had been feared, but ends with
agreements on a number of issues.
2011 - A new
analysis of the Earth's temperature record by scientists concerned over the
"ClimateGate" allegations proves the planet's land surface really has
warmed over the last century.
2011 - Human
population reaches seven billion.
2011 - Data
shows concentrations of greenhouse gases are rising faster than in previous
years.
2012 - Arctic sea ice reaches a minimum extent of 3.41
million sq km (1.32 million sq mi), a record for the lowest summer cover since
satellite measurements began in 1979.
2013 - The
Mauna Loa Observatory on Hawaii reports that the daily mean concentration of
CO2 in the atmosphere has surpassed 400 parts per million (ppm) for the first
time since measurements began in 1958.
2013 - The
first part of the IPCC's fifth assessment report says scientists are 95%
certain that humans are the "dominant cause" of global warming since
the 1950s.
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